Photodetectors are used in a variety of instruments where amount of light is to be measured. One class of detectors provides periodic readouts of light accumulated during sequential time intervals. These detectors are well known and generally are solid state, based on the principles of charge generation upon the incidence of radiation on a surface such as doped silicon. To provide image or spectral resolution the surface is divided into pixel areas. The accumulations and readouts of signals from the pixels are effected through the transfer of charges from the pixels, with readouts repetitively cycled through the pixels.
The solid state photodetectors are quite accurate in linearity of signal strength versus amount of light collected in a readout period. The amount of light may be variously defined as the total number of photons collected, or the light intensity integrated over the period. However, linearity is not perfect and calibration is necessary for levels of precision needed in some instruments, for example an absorption spectrophotometer for analysis of liquids.
Calibrations have been effected in various ways such as by varying intensity of a source lamp, but this may not provide the precision required. A conventional method of calibration is to vary the readout period of the detector. However, changing the readout period of self scanned array detectors may in itself cause changes in detector responsivity.
Another method utilizes multiple apertures, allowing the light intensity to be measured through each one and then in combination. By taking the ratio of the combined intensity to the sum of the individual ones, a degree of nonlinearity can be determined. A newly implemented method based on a similar scheme is disclosed in "Automated High Precision Variable Aperture for Spectrophotometer Linearity Testing" by J.C. Zwinkels and D.S. Gignac, Applied Optics, 30, 1678-1687 (May 1, 1991). This requires high precision PZT motors driven by a dedicated controller under microcomputer direction.